The present invention relates to a cracking process of diorganopolysiloxanes to produce a cyclotrisiloxane therefrom and more particularly the present invention relates to the above cracking process where there is incorporated along with the reactants an effective amount of a higher aliphatic alcohol so that the yield of cyclotrisiloxane of improved purity from the cracking process is over 90% by weight and is obtained at a faster rate. Heat cured silicone rubber compositions are well known in the silicone art. Such heat cured compositions comprise a linear diorganopolysiloxane polymer having a viscosity varying from 1,000,000 to 200,000,000 centipoise at 25.degree. C. where the organo groups are selected from monovalent hydrocarbon radicals and halogenated monovalent hydrocarbon radicals, extending or reinforcing fillers such as silica, lithopone, zinc oxide, etc. pigments, heat aging additives, flame retardant additives, vulcanizing agents and various other additives.
Such heat curable silicone compositions are cured by incorporating into them an organic peroxide, which upon heating the composition at temperatures above 100.degree. C. results in a silicone elastomer. Silicone oils are also well known in that they comprise linear diorganopolysiloxane polymers of a viscosity of anywhere from 50 to 100,000 centipoise at 25.degree. C. Various other additives may be added to the composition.
There have also been developed room temperature vulcanizable silicone rubber compositions wherein the main ingredient is silanol terminated diorganopolysiloxane polymer having a viscosity varying anywhere from 1,000 to 500,000 centipoise at 25.degree. C. where the organo groups are selected from monovalent hydrocarbon radicals and halogenated monovalent hydrocarbon radicals. Such compositions include various ingredients in them besides the silanol fluid, such as adhesion promoters, the extending and reinforcing fillers, cross-linking agent such as methyltriacetoxysilane or methyltrimethoxy silane and catalysts such as a metal salt of a carboxylic acid. These compositions cure to a silicone elastomer at room temperature upon being exposed to atmospheric moisture.
Also, there have been developed silicon hydride olefin platinum catalyzed silicone rubber compositions which comprise vinyl terminated diorganopolysiloxane polymers of a viscosity varying from 1,000 to 200,000,000 centipoise at 25.degree. C.; silicon hydrides containing polysiloxane or silicone resin; and a platinum catalyst. Such compositions when all the components are mixed together will cure at room temperature over an extended period of time or in a matter of a minute at temperatures above room temperature to a silicone elastomer.
The most basic ingredient in any of these compositions, is a linear diorganopolysiloxane polymer, which may have any of the viscosities mentioned above. Accordingly, the process for producing such linear diorganopolysiloxane polymers is one that has merited a great deal of attention. For most organo groups in such polymers, that is where the organo groups are selected from lower alkyl such as methyl, ethyl, propyl and vinyl. Such processes for producing such linear diorganopolysiloxane polymers comprise first adding diorganodihalogensilanes to water. Such diorganodichlorosilanes may contain up to 10% of trifunctional silane units or monofunctional silane units. The resulting diorganodichlorosilanes when added to water hydrolyze to form a mixture of cyclic siloxanes, that is cyclotrisiloxanes and cyclotetrasiloxanes and up to cyclosiloxanes containing 10 or more siloxy units. The silicone hydrolyzate mixture also contains low molecular weight silanol terminated diorganopolysiloxanes where the number of diorganosiloxy units is repeated anywhere from 1 to 30 times.
Such a silicone hydrolyzate is of little use by itself to form any of the foregoing polymers mentioned previously. Accordingly, it is common in the silicone industry to take such a silicone hydrolyzate and add to it an alkali metal hydroxide at a concentration of anywhere from 5 to 500 parts per million of the silicone hydrolyzate. The alkali metal hydroxide normally being potassium hydroxide. The mixture is then heated at elevated temperatures above 100.degree. or 150.degree. C. so as to convert most of the silicone hydrolyzate to distilled cyclotetrasiloxanes; such cyclotetrasiloxanes being the desired ingredient for the preparation of linear diorganopolysiloxane polymers. It is also known that if the hydrolyzate is carefully fractionated during the heating process, then cyclotrisiloxane can be distilled off during such a cracking process and most of the hydrolyzate will be converted to the cyclotrisiloxane. However, if the cyclotrisiloxane is not removed and the silicone hydrolyzate is heated and distilled rapidly, then most of the silicone hydrolyzate will be converted to cyclotetrasiloxanes. Such cyclotetrasiloxanes can be mixed with cyclotetrasiloxanes substituted by other organic groups, if desired, which siloxanes are produced by the same cracking reaction and the resulting mixture equilibrated at temperatures above 100.degree. or 150.degree. C. and preferably below 150.degree. C. in the presence of an alkali metal hydroxide catalyst preferably potassium hydroxide, which is desirably present at a concentration of 5 to 500 parts per million to convert most of the cyclotetrasiloxanes to a linear diorganopolysiloxane polymer.
There is also included in such a reaction mixture the appropriate amount of chainstoppers such as hexamethyldisiloxane or divinyltetramethyldisiloxane or other triorganosiloxy endstopped diorganopolysiloxanes of low molecular weight so as to chainstop the linear diorganopolysiloxane polymer. The amount of chainstoppers generally determine the ultimate or average molecular weight of the linear diorganopolysiloxane polymers that is formed. It should be noted that during such a reaction the most conversion of the linear diorganopolysiloxane from the cyclotetrasiloxanes is obtained is when 85% (in the case of a dimethyl substituted siloxane) of the cyclotetrasiloxanes have been converted to the linear diorganopolysiloxane polymer. When this 85% equilibrium point has been reached or approached, it is common to neutralize the basic catalyst in the reaction mixture and vent off the remaining cyclotetrasiloxanes to yield the pure diorganopolysiloxane polymer.
It should be noted that in the case of low molecular weight linear diorganopolysiloxane polymers that are formed the equilibration reaction is carried out in the presence of acid catalysts such as, toluenesulfonic acid.
In the production of low molecular weight linear diorganopolysiloxanes, it is common to use an acid catalyst while in the production of higher viscosity diorganopolysiloxane polymers it is more practical to use strong basic catalysts.
As can be appreciated from the above, the cracking reaction for forming the maximum amount of cyclotetrasiloxanes is very important in the formation of such linear diorganopolysiloxane polymers.
It should also be noted that the production of fluorine substituted linear polysiloxane polymers are important in producing any of the above polymer compositions, the advantage of fluorine substituted linear polysiloxane polymers being to prepare solvent-resistant silicone elastomers. However, the formation of fluorine or fluoroalkyl substituted linear polysiloxane polymers proceeds within an equilibration reaction utilizing a cyclotrisiloxane instead of the traditional cyclotetrasiloxanes; for instance see the disclosures of U.S. Pat. No. 2,961,425 and U.S. Pat. No. 2,979,519, U.S. Pat. No. 3,002,951 and U.S. Pat. No. 3,179,619.
In fluorosilicone chemistry, the process for forming fluoroalkyl substituted siloxanes comprises hydrolyzing fluoroalkyldichlorosilanes, adding an alkali metal catalyst to the fluorosilicone hydrolyzate and cracking the mixture at elevated temperatures of above 100.degree. and more preferably at reduced pressures and continuously during such cracking reaction stripping off cyclotrisiloxanes as they are formed. By stripping off the cyclotrisiloxanes as they are formed, the yield of the cyclotrisiloxanes from the silicone hydrolyzate is maximized. In accordance with this process, it is possible to obtain 80% conversion of the silicone hydrolyzate to cyclotrisiloxanes and some times even as high as 90% or more by weight of the silicone hydrolyzate. The cyclotrisiloxanes are then equilibrated with the appropriate amount of chainstoppers with a basic catalyst as disclosed in the foregoing patents to obtain the desired molecular weight linear fluoroalkyl substituted polysiloxane polymer at about 100% conversion of the cyclotrisiloxanes to the linear polymer. Then the catalyst is neutralized and the linear polymer can be utilized in production of fluorosilicone compositions.
It should be noted that there are other processes for producing linear fluorosilicone polymers from cyclotetrasiloxanes, such as for instance those disclosed in U.S. Pat. No. 3,997,496 and U.S. Pat. No. 3,937,684 of John S. Razzano which are hereby incorporated into the present case by reference. However, these fluorosilicone cyclotetrasiloxanes when they are reacted in an equilibration reaction produce above 60% yield of the linear polymer but below 100% since such a process is not as efficient for the production of fluorosilicone polymers as is the case of the cyclotrisiloxanes.
Accordingly, to maximize the efficiency and to minimize the cost of the production of fluorosilicone composition, it is necessary to maximize the efficiency of the cracking reaction so as to obtain the maximum amount of cyclotrisiloxane from the cracking reaction of the fluorosilicone hydrolyzate, since in the equilibration step where 100% of the cyclotrisiloxane is converted to the linear polymer, it is not seen that part of the process can be maximized any further. However, in the process of producing the fluorosilicone cyclotrisiloxane by the cracking of the silicone hydrolyzate with a basic catalyst, it has been found normally that as high a yield of the cyclotrisiloxane is not obtained as could be desired. Accordingly, yields of 80% and sometimes as high as 90% have been achieved if the process conditions were followed meticulously. However, in many cases, there is only obtained 70% yields or less and in addition, the cyclotrisiloxane, that is obtained has a small amount of impurities in it which are undesirable in the finished silicone elastomer. Since such impurities are liquids and soluble they cannot be filtered out. In addition, when the silicone hydrolyzate when 70% of it or more has been converted to the cyclotrisiloxane the residue has the tendency to brown and gel, thus becoming unusable. In addition, many times a strong objectionable odor is imparted to the cyclotrisiloxanes that are distilled in the cracking reaction which odor has yet been unidentified.
Accordingly, it is highly desirable in the production of fluorosilicone polymers to maximize the yield and rate of formation of cyclotrisiloxanes from the cracking reaction of the silicone hydrolyzate and to minimize the amount of the impurities that may be distilled over with the cyclotrisiloxane.
Accordingly, it is one object of the present invention to provide for an efficient process for producing fluoroalkyl cyclotrisiloxanes.
It is an additonal object of the present invention to provide for an improved process for producing fluoroalkyl cyclotrisiloxanes by using in addition to the cracking catalyst an effective amount of a higher alcohol.
It is another object of the present invention to provide for an improved process for producing fluoralkyl cyclotrisiloxanes at a yield of more than 80% or more in the cracking reaction of the silicone hydrolyzate.
It is yet an additional object of the present invention to provide for an improved process for producing fluoroalkyl cyclotrisiloxanes from silicone hydrolyzates where the cyclotrisiloxanes have very few impurities in it. These and other objects of the present invention are accomplished by means of the disclosure set forth hereinbelow.